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57ISSN 2072-5981
Volume 18,
Issue 2 Paper No 16204,
1-6 pages 2016
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"Magnetic Resonance in Solids. Electronic Journal" (MRSej) is a
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Hugo Keller (University of Zurich, Zurich) Yoshio Kitaoka (Osaka University, Osaka) Boris Malkin (KFU, Kazan) Alexander Shengelaya (Tbilisi State University, Tbilisi) Jorg Sichelschmidt (Max Planck Institute for Chemical Physics of Solids, Dresden) Haruhiko Suzuki (Kanazawa University,
Kanazava) Murat Tagirov (KFU, Kazan) Dmitrii Tayurskii (KFU, Kazan) Valentin Zhikharev (KNRTU, Kazan)
*
In Kazan University the Electron Paramagnetic Resonance (EPR) was discovered by Zavoisky E.K. in 1944.
E.M. Alakshin1, A.V. Klochkov1, S.L. Korableva1, V.V. Kuzmin1, D.S. Nuzhina1’*,
I.V. Romanova1, A.V. Savinkov1, M.S. Tagirov1’2 1Kazan Federal University, Kremlevskaya 18, Kazan 420008, Russia 2Institut of Perspective Research, TAS, L. Bulachnaya 36a, Kazan 420111, Russia
*E-mail: nuzh.darya@gmail.com
(Received December 10, 2016; accepted December 17, 2016)
Samples LiTbF4 and TbF3 were synthesized by modified methods of colloidal chemistry. The magnetization of these samples was measured in the external magnetic field at 100 Oe and 1 T and in temperature range 2-300 K. Temperatures of phase transition to the magnetic ordering dipolar ferromagnet state were determined for synthesized samples.
PACS: 75.50.-y, 75.90+w, 81.10-h
Keywords: synthesis, magnetization, Curie temperature
1. Introduction
The complex fluorides LiReF4 (Re is rare earth ion) represent a class of crystal materials used as model objects in physics of dipolar magnets. Crystals of rare-earth tetrafluoride compounds with a controlled size, shape, structure and surface have unique optical, electronic, magnetic and catalytic properties important for practical applications [1]. The LiTbF4 crystal has a scheelite (CaWO4) structure with the space group C4h [2] and TbF3 crystallizes in orthorhombic D\h space group [3]. Trifluorides exhibit distinct magnetic properties at low temperatures and they are of interest as model systems for the theoretical study of magnetic ordering in condition of competition between the dipole-dipole and exchange interactions [4].
Measurements of magnetic DC-susceptibility in LiTbF4 single crystals showed that LiTbF4 is the dipole Ising dielectric ferromagnet at temperatures below the Tc = 2.89K [5,6]. Magnetic moments of Tb3+ are ordered along the c-easy axis at T < Tc. The TbF3 is also a dipolar ferromagnet, as it was found in the magnetization measurements [4]. The phase transition in TbF3 single crystal from the paramagnetic to the ferromagnetic state occurs at temperatures below Tc = 3.95 K [4]. At low temperatures (T < Tc) magnetic moments of ions Tb3+ ( 9 iaB) are ordered in two magnetically non-equivalent sublattices in the crystallographic ac plane at angles ф = ±28° to the a-axis. Due to large values of the magnetic moments, the magnetic ordering is induced mainly by classical dipole-dipole interaction between the terbium ions, which dominates the magnetic exchange interaction.
The tetrafluorides LiTbF4, LiDyF4, LiHoF4 and LiErF4 have been the subject of intensive studies by NMR and EPR. The nuclear magnetic resonance rotation spectra were observed and contributions to the local magnetic fields from the dipole and the transferred hyperfine interactions were distinguished [7]. The fluorine and lithium NMR line shifts have been measured [8] in the temperature range from 300 to 1.3K and in the fields up to 40kG for LiTbF4, LiHoF4. Angular dependences of fluorine NMR spectra have been observed in the external magnetic field,
^This paper material was selected at XIX International Youth Scientific School “Actual problems of magnetic resonance and its application”, Kazan, 24 - 28 October 2016. The paper was recommended to publication in our journal and it is published after additional MRSej reviewing.
Figure 1. XRD patterns of samples 1-3.
Table 1. Characteristic size of samples 1-3 obtained by Photocor Complex.
Sample Characteristic size (diameter)
LiTbF4 (79.9%)+TbF3 (20%) 20 mkm
LiTbF4(6.9%)+TbF3(81.2%) 1.5 mkm
TbF3 585 nm
the constants of transferred hyperfine interaction and the corrected set of crystal field parameters for the Tb3+ ions in LiTbF4 have been determined determined [9]. The EPR investigations were done in LiTbF4, LiHoF4 and LiErF4 single crystals at submillimeter wavelength. The temperature and angle dependencies of resonance line width were studied and described by the dipole dipole interactions of Re3+ ions and local fields around Re3+ ions [10]. The hyperfine structure of EPR spectra of Tb3+ ions was considered [11], the strong dipole interactions give a strong line broadening, but if the applied magnetic field is high enough to saturate the magnetization, each electron becomes in the same magnetic surrounding, so the line narrowing one can observe.
Influence of nanoparticle size on the magnetic, optical and electronic properties of the fluorides is one of the fundamental problems in the physics of the rare-earth compounds and fundamental physics. Nanoparticles of DyF3 were synthesized by the colloidal chemistry methods [12]. The influence of the microwave-assisted hydrothermal treatment on magnetic properties and structure of PrF3 and LaF3 nanoparticles was investigated by XRD, TEM, NMR and EPR [12-21]. In this paper we present the few synthesis technique for TbF3 and LiTbF4 materials. Magnetic and structure properties of the TbF3 and LiTbF4 powders are investigated by DC-susceptibility measurements.
2. Syntesis of micro- and nano-sized powders
The synthesis of LiTbF4 is complex procedure [1,22]. The method of synthesis with trifluroac-etate precursors is chemically aggressive, the methods of hydrothermal synthesis and thermal decomposition demand high temperatures and high pressure in autoclave [22-28].
The samples of LiTbF4 were synthesized using following techniques:
Sample 1: LiNO3(aq)+Tb(NO3)3+NH4(aq)4powder0004o44LiTbF4(s)+TbF3(s).
Sample 2: LiOH(aq)+Tb(NO3)3+NH4(aq)4powder 400 ’ h ’ C F4>LiTbF4(s)+TbF3(s).
The TbF3 nanoparticles were synthesized using standard technologies [10,11].
Sample 3: Tb(NO3)3+NaF(aq)4TbF3(s)+NaNO3(aq).
The X-ray analysis was carried out by an automatic X-ray diffractometer Bruker D8 Advance with Cu Ka radiation (A = 1.5418 A) at Bragg-Brentano geometry. Result of the X-ray analysis of synthesized samples is shown in Fig. 1. Phase ratio of LiTbF4 and TbF3 in samples 1-2 was determined using MAUD software [29].
Figure 2. Derivatives of magnetic susceptibility versus temperature for LiTbF4(79.9±0.7%) +TbF3(20.0±0.5%) powders (black points) and LiTbF4 single crystal (red points) [30] in external magnetic field 100 Oe.
Figure 3. Temperature dependencies of magnetic susceptibility for sample 2 (TbF3(81.2±0.4%) +LiTbF4(6.9±0.1%)) (black points), sample 3 (blue points) and micropowder TbF3 (red points) in external magnetic field 100 Oe.
The chemical composition of the synthesized samples is:
Sample 1 - LiTbF4(79.9±0.7%)+TbF3(20.0±0.5%),
Sample 2 - TbF3 (81.2±0.4%) + LiTbF4(6.9±0.1%)+LiNO3 H2O(11.9±1.1%),
Sample 3 - TbF3(99±1%).
Particle size was preliminary determined using the dynamic and static light scattering spectrometer for registration nanoparticles size and composition Photocor Complex (table 1). Such big particles size probably can be explained by agglomeration of particles in the solution.
3. Study of magnetic properties, discussion and results
The temperature dependencies of magnetic susceptibility for all synthesized samples were measured by PPMS-9 System (Quantum Design Inc.) at applied magnetic fields 100 Oe and 1 T in temperature range 2-300 K. The derivative of magnetic susceptibility versus temperature for the sample 1 is shown in Fig. 2. There are two phase transitions at temperatures T = 2.9 K and T = 3.93 K, which correspond to the Curie temperature of the transitions to the ferromagnetic state for LiTbF4 and TbF3 single crystals, respectively. The red curve in Fig. 2 represents the dependence for single crystal LiTbF4 in the magnetic field B||c [30]. The total magnetization would be reduced, because all possible particles directions in external magnetic field are equivalent in powder.
The temperature dependences of magnetic susceptibility for the sample 2 (black points) and sample 3 (blue points) are shown in Fig. 3. There are no clear evidence of transitions to a magnetically ordered state for TbF3 or LiTbF4. This may be due to the fact that the ordering of magnetic domains happens not simultaneously.
Fig. 4 shows the measured temperature dependence of the magnetic susceptibility of the powder samples 3 (TbF3 powder). The temperature dependences of the magnetic susceptibility of single crystal TbF3 demonstrate a strong anisotropy (the black points correspond to the %||a direction, the pink points correspond to the %||6 direction, the green points correspond to the x||c direction) [4].
The magnetic susceptibility of the powder can be described by xP = (xa+хЪ+Xc)/3, where Ха, Хъ Xc are magnetic susceptibilities along the axes a, b, c due to the symmetry of the crystal lattice. These calculations do not well fit the experimental results, because the size and shape of the particles were not taken into account. The crystal field and local magnetic field at the rare-earth ions located near the surface of nanoparticles are also different from ones in the bulk.
Theoretical calculation of the magnetic susceptibility of LiTbF4 is shown on Fig. 5 (red line). Here all possible orientations of the external magnetic field along the crystallographic c-axis of the nanoparticles were taken into account. The shape of the nanoparticles assumed spherical, the summation carried on sphere 360°. Therefore the magnetization along c-axis [30]:
Mz = Bz
4
tanh(5/2kpTc) tanh(5/2kBT)
l) + N
where A is molecular field constant (A = 5.23 ± 0.56), 5 is initial splitting in the ground state quasidoublet (5 = lcm_1) of basic 7F6 multi-plet, N is demagnetization factor (N = 4^/3), Bz is external magnetic field, Tc is the Curie temperature.
The measured temperature dependences of sample l and single crystal LiTbF4 at Bo У a and B0 Ус in external magnetic field (B0 = lT) are presented in the Fig. 5.
The curve 5 in Fig. 5 shows the calculations which take into account all possible crystallographic c-axis of the particle orientations with respect to external magnetic field. The discrepancy between the theoretical calculations and experimental data can also be explained by the fact that the calculation has not considered TbF3 presence, nanoparticles shape and size.
Figure 4. Temperature dependencies of magnetic susceptibility for sample 3 (cyan points), micropowder TbF3 (red points), single crystal TbF3 in different directions of crystal lattice [4] and calculation magnetic susceptibility of the powder TbF3 [4] (blue points).
T-1, K-1
Figure 5. The temperature dependence of the magnetization for sample l (LiTbF4(79.9±0.7%) +TbF3(20.0±0.5%)) (curve l, black points), LiTbF4 single crystal along the crystallographic c-axis (curve 2, black points) [30] and a,b-axis (curve 3, black points) [30]; Theoretical calculations of the magnetization for LiTbF4 single crystal along c-axis (curve 2, black line) [30], calculations of the magnetization of LiTbF4 (curves 4, 5) at B = l T.
Curve 4 (blue line) in Fig. 5 shows the calculations which take into account the following considerations: magnetic Tb3+ moments in LiTbF4 single crystal are arranged along c-axis of the crystal lattice, the magnetization along the a-axis and b-axis is almost zero. Distribution of the c-axis orientations related to the direction of the external magnetic field is of the same probability. Thus, the magnetization along the a and b-axis can be neglected and the total magnetization can be estimated as M = Mz/3. The unusual behavior of the magnetization and magnetic susceptibilities in micro- and nano-sized powders LiTbF4 and TbF3 will be studied in detail later.
4. Conclusion
The samples of the rare-earth fluorides were synthesized. Crystallographic structure of the synthesized samples has been identified by the X-ray diffraction, magnetic properties of the powders mainly agree with the LiTbF4 magnetic properties in the bulk. However improvement of the synthesis method is needed because the undesirable TbF3 phase (~20%) was found.
Acknowledgments
The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University, partly supported by the Russian Foundation for Basic Research (Project No.16-32-60155 mol_a_dk). The magnetic measurements were carried out at the Federal Center of Shared Facilities of Kazan Federal University. Authors thank Shustov V.A. for analysis XRD patterns.
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